Traditionally, plastics have been formulated to result in strong, light-weight, durable, and bioresistant polymeric materials. It is the durability and indestructibility that makes plastic the material of choice for many applications. However, these same properties are problems when the plastics enter the waste stream. The recent trend is to create biodegradable plastics, most of such plastics being first commercialized in the mid 1980's..sup.1
Among the first biodegradable plastics made were blends of non-biodegradable polyolefins with starch which were at best only partially biodegradable. These plastics are not compatible with waste management infrastructures such as composting. Moreover, at that time the appropriate infrastructures capable of dealing with biodegradables did not exist. Instead of composting, these products generally ended up in sanitary landfills.
Landfills, in general, are a poor choice as a repository of plastic and organic waste. The landfills are plastic-lined tombs designed to retard biodegradation by providing little or no moisture with negligible microbial activity. Organic waste, such as lawn and yard waste, paper, and food waste should not be entombed in such landfills to be preserved for posterity. Accordingly, there is a growing trend to divert these materials into composting facilities which allow them to be biodegraded to produce humus or compost. This compost can then be used as a valuable soil additive for new plant growth.
When plastics are designed to be biodegradable, utilizing renewable resources as the major raw material component, the plastics can become part of an ecologically sound mechanism.
Biodegradation of such natural materials produces valuable compost as the major product, in addition to water and carbon dioxide. Such carbon dioxide is fixed, or neutral carbon dioxide, and therefore does not contribute to an increase in the greenhouse gases.
Rowell, Schultz and Narayan published a book on the emerging technologies from materials and chemicals from biomass..sup.2 In the first chapter of that book, Narayan discusses the need for environmentally compatible polymers based on renewable resources. In that book is included a discussion of tailor-made cellulose-polystyrene graft polymers which were used as compatibilizers/interfacial agents to prepare cellulosic-polystyrene alloys and wood-plastic alloys (Ch. 5, pp 57-75)..sup.2 The graft copolymers function as emulsifying agents and provide for a stabilized, fine dispersion of the polystyrene phase in the continuous phase of the cellulosic matrix.
U.S. Pat. No. 4,891,404 to Narayan et al..sup.3 discusses a specific nucleophilic displacement reaction used to prepare such graft polymers which are disclosed to be biodegradable thermoplastic copolymers exhibiting a high capacity for stabilized biodegradable blends of polysaccharides and synthetic thermoplastic polymers. The patent discusses the problems relating to the making of cellulose/starch natural biopolymers and the problem of controlling the molecular weights and degree of substitution of such polymers. Earlier papers by Narayan and Stacy et al..sup.4,5 further discuss biodegradable natural/synthetic graft copolymers.
U.S. Pat. No. 5,095,054 to Lay et al.,.sup.6 issued Mar. 10, 1992, discloses the use of water as a plasticizer for starch (referred to as starch "destructurization") in order to make the material processable in for example an extruder. Products derived therefrom, tend to have the problem of rapidly losing water to the environment by evaporation. As a result this type of material tends to become brittle with age. These materials are also highly water sensitive which is undesirable for the majority of applications of thermoplastic products.
To address this issue of water sensitivity, the patent includes various blends of destructurized starch with a variety of synthetic petroleum-based plastics. These lends, along with the earlier starch-filled polyolefins, are at best only biodisintegratable and not fully biodegradable..sup.12,31-33 Similar starch-polyolefin compositions have been reported by the Fertec group..sup.7
The U.S. Pat. No. 4,863,655 to Lacourse et al.,.sup.8 issued Sep. 5, 1989, discloses water-soluble high amylose starch based compositions containing poly(vinyl alcohol). This biodegradable modified starch product intended for loose fill, or "peanut-shell"-type foam packaging applications, for example, contains a hydroxy propylated starch having a very low degree of substitution. This type of modified starch is highly hydrophilic and water soluble; the starch contains about 5% by weight propylene oxide corresponding to a theoretical degree of substitution of 0.19. This is a very low degree of substitution compared with the maximum degree of substitution for starch which is 3.0 according to the three available hydroxyl groups on the anhydroglucose repeat unit. The poly(vinyl alcohol) typically used as a blend component further adds to the water-sensitive nature of these materials. In the case of peanut-shell packaging, the water solubility of such starch-based foams is in fact a positive as this allows the material to be disposed of in an environmentally friendly fashion by simply washing them with water down the drain; the material subsequently biodegrades in the sewer system. For other applications, however, which utilize moldable compositions for various packaging applications, fast food cutlery, plates, cups, etc., the need for moisture resistance is of ultimate importance.
The prior art on biodegradable materials is restricted to starch-based materials in which the starch component is hydrophilic (water sensitive). No prior art exists on making hydrophobic, thermoplastic modified starches as fully biodegradable products which are readily processable on conventional plastics processing equipment such as extruders, injection molders, etc. There are a number of patents and publications in the literature relating to modification of starch by esterification and etherification reactions. Most commercial modified starch products have low degree of substitution (DS) levels, are generally made by reactions in water with excess anhydride, and are designed to alter their solution properties for food applications or adhesion to paper. Acetylated starches, for example, have been known for more than 100 years. Starch acetates ranging from about 0.3 to about 1 DS are typified by water solubility..sup.9 Starch esters which are commercially available for consumption, used for example in salad dressings, have a degree of substitution which typically is lower than 0.1 DS. For example, starch succinate derivatives are cleared for food use by the U.S. Food and Drug Administration (FDA) up to a 4% treatment level, which is equivalent to 0.07 DS..sup.10
Highly acetylated starches, historically, were of some interest because of their organic solvent solubility and their thermoplasticity for film and fiber applications analogous to thermoplastic cellulose esters. In spite of this early development, high DS starch esters have not been developed commercially because they could not compete with similar cellulose derivatives in terms of strength and cost..sup.9 Of primary focus were starch triesters, which fell short in strength and impact properties..sup.11,12 Such high-DS starch esters are characterized by their crystalline properties exhibiting clear melt transitions..sup.13 These high-DS starch esters are not biodegradable. Rivard et al. showed that under anaerobic conditions starch esters above substitution levels of DS=1.7 were not biodegradable..sup.14 We have obtained similar results in our laboratory under composting conditions.
In the present invention, we have designed starch esters with the appropriate degree of substitution, prepared by a unique homogeneous base-catalyzed system under anhydrous conditions, that allows us to obtain starch ester compositions having good mechanical properties while maintaining complete biodegradability. This requires starch ester compositions described in the present invention, to have an intermediate degree of substitution, preferably ranging from 0.4 to 2.5 DS, more preferably from 1.0 to 2.0, and most preferably from 1.2 to 1.7 DS. The latter range of compositions have the most preferred balance in mechanical properties, water resistance, processability and the rate of biodegradation. The starch esters prepared by the present invention are predominately amorphous polymers; little or no residual native starch crystallinity remains due to the homogeneous modification process employed. Without being restrictive, the absence of a new crystalline structure for the starch esters produced by this process relates to the range of intermediate degrees of substitution to give non-crystalline copolymers. High DS starch triesters approach the structure of a homopolymer having the needed macromolecular chain regularity required for crystallization. By designing starch esters of intermediate degree of substitution, prepared in a homogenous modification process, the placement of ester groups on the anhydroglucose repeat units is expected to follow a statistically random distribution pattern. This results in irregular macromolecular chains, giving rise to novel amorphous thermoplastics with unique properties.
In addition, most high DS starch ester preparations in the prior art,.sup.15-22 involve aqueous heterogeneous systems with excess anhydride, resulting in broad DS distribution profiles and therefore poor mechanical properties and poor moisture resistance. The economics of such processes are unfavorable due to hydrolysis of the anhydride, whereas high yields are obtained in the present invention which employs anhydrous conditions. The use of organic solvents such as organic acids,.sup.23 or dimethyl sulfoxide with sulfuric acid catalyst,.sup.24 is also reported in the prior art, although molecular weight breakdown of the starch and starch ester products is inevitable under those conditions..sup.25,26
The present invention achieves good processability and mechanical properties by controlling the degree of substitution and molecular weight of the product. When anhydride is used as the esterification reagent, molecular weight breakdown of starch and starch esters is minimized by addition of a neutralizing agent to the reaction mixture. The control of molecular weight is achieved by neutralization of the acid by-product throughout the reaction in an excess of the neutralizing agent. This is in contrast with prior art high DS starch esters which either yielded degraded products.sup.25,26 having significantly reduced molecular weights and consequently reduced mechanical properties, or utilized processes which are commercially unattractive due to high solvent recovery costs as well as issues regarding toxicity and safety as in the case of pyridine..sup.16,25-29 In another method, claimed to be commercially more attractive than the pyridine-based process described the art prior to 1972, a 212% molar excess of acetic anhydride was used to acetylate starches to high substitution levels..sup.15,30 In contrast, in the present invention high yields (up to 96%) are obtained using equimolar levels of anhydride relative to starch hydroxyl moieties. Without being restrictive, this improvement is attributed to the highly effective acylation catalyst and anhydrous conditions. Prior art.sup.15-30 reveals significant hydrolysis of the anhydride reagent to occur due to the relatively high water levels present during the reaction. Furthermore, the absence of incomplete starch granule destruction, as clearly evidenced in the literature,.sup.30 indicates that these types of processes result in heterogeneous starch substitution. The homogeneous base-catalyzed process employed in the present invention, affords starch esters having a more uniform and narrow substitution profile than prior art.
This control of substitution and molecular weight results in starch ester compositions having good moisture resistance and excellent mechanical properties, not obtained with the previous inventions.
A key aspect of this invention is that these compositions are fully biodegradable (complete mineralization), as opposed to blends of biodegradable starch compositions with conventional petroleum-based plastics described earlier..sup.12,31-33 As discussed, such blend compositions are at best biodisintegratable and not fully biodegradable. In composting, the non-biodegradable components will be persistent resulting in an irreversible build-up of these components in the environment causing reduced productivity and fertility of the soil..sup.34 Even if such "biodegradable" blend compositions, described in the prior art, are partially biodegradable, the resulting compost will have very little value. In fact, these recalcitrant components will be present in the final compost at significantly higher concentration levels than in the original waste mixture..sup.34